When a plant is not soluble in standing cold or boiling hot water, it will often yield its soluble ingredients by simmering in almost boiling water for 30 minutes or more. This simmering of roots, barks, resins, rhizomes, and berries is called decoction. In effect, you are making a soup stock from a tough plant substance.
For decoctions, fresh herbs should be sliced; dry herbs should either be powdered or well bruised.
Decoctions should always be strained while hot, so that the matter which separates on cooling may be mixed again with the fluid by shaking when the remedy is used.
Use glass, ceramic, or earthenware pots, or clean, unbroken enameled cast iron. Do not use plain cast iron with astringent plants.
Examples of plants to be decocted: elm bark, uva ursi, barley, flaxseed, broom, quince seed, comfrey root, cherry bark, oak bark, and some gum resins. You may preserve decoctions in the same way you preserve infusions.